I didn't want to make things too complicated, so I focused on the adoption by nation states in my original post. The use of the Julian calendar in todays Serbian Orthodox Church you mentioned is a special of a broader movement. Some other Orthodox Churches use it as well. In a sense, the only territory that AFAIK uses the Julian calendar today is the Monastic Republic of Mount Athos[1], which is an autonomous self-governed territory inside Greece.
[1] https://en.wikipedia.org/wiki/Monastic_Republic_of_Mount_Ath...